1. Field of the Invention
This invention relates to the isolation of pentabromophenol and more particularly to a process for isolating pentabromophenol containing residual free bromine and by-product hydrogen bromide which avoids the formation of substantial quantities of undesirable impurities.
2. Description of the Prior Art
Numerous processes may be employed for replacing all the nuclear hydrogen atoms in aromatic compounds such as benzene, phenol and the like. Perbromination processes in general have involved use of up to about 20% excess bromine in the presence of various kinds of reaction media and solvents depending on the particular aromatic compound to be brominated.
Alternatively, nuclear perbrominated non-condensed aromatic compounds may be preferably obtained by reacting such a compound in the presence of a substantial excess of bromine as both reactant and the sole reaction medium. This bromination in bromine reaction takes place in the presence of a small but catalytic amount of a bromination catalyst which may include iron and aluminum, their halides, and compounds which form iron or aluminum bromide under the conditions of the reaction.
Regardless of the preparation process utilized there is obtained a perbrominated compound containing undesirable large amounts of occluded free bromine and by-product hydrogen bromide.
High levels of purity are often required for perbrominated aromatic compounds such as pentabromophenol which have found utility as flame retardant agents. In particular, it is important that such brominated products have extremely low levels of residual impurities such as free bromine, bromine-containing derivatives and the like since the presence of such impurities can have undesirable effects on the compositions in which such compounds are used as either flame retardant agents or for other purposes. Purity is particularly important from the standpoint of color and thermal stability under the processing conditions to which such compounds are subjected.
The isolation of perbrominated products obtained from the bromination in bromine reaction has been attempted by a variety of work-up procedures.
The crude reaction mixture which may contain the brominated products, excess bromine, hydrogen bromide, and catalyst can, for instance, be subjected to stripping either at atmospheric pressure or preferably under reduced pressure at about 80.degree. C. to the point of constant weight of the residue. The crude product which is thus isolated may be further purified, for instance, by digestion with methanol, ethylene dibromide, or dilute hydrochloric acid. This method has the disadvantage that it may leave residual metal-containing catalyst in the product and is impractical on anything other than a laboratory scale.
It is also possible to replace the excess bromine in the reaction mixture by ethylene dibromide followed by filtration. The crude product can then be further purified by digestion procedures as described above. However, the substitution of excess bromine with ethylene dibromide is inconvenient because the two distlll together and the resulting volume of distillate containing bromine and ethylene dibromide can be more than twice as large as the volume of excess bromine used. Furthermore, this isolation seals in the residual metal-containing catalyst, and the catalyst remains in the product even after digestion in methanol.
In a preferred variation of the isolation step it is also possible to isolate the perbromination product by replacing the excess bromine in the reaction mixture with water while distilling off the excess bromine to yield a water slurry of perbrominated product. This is then followed by filtration, washing and drying of the aqueous reaction mixture. The crude product obtained may then be purified by relatively simple digestion procedures as described above.
However, when applied to the isolation of pentabromophenol, the removal of excess bromine by transferring the reaction mixture into contact with water results in the formation of substantial quantities of the carbonyl impurities 1, 2, 4, 5, 6, 6-hexabromo-1, 4-cyclohexadien-3-one, and 1, 2, 4, 4, 5, 6-hexabromo-1, 5-cyclohexadien-3-one (hereinafter referred to as "carbonyl impurities" or "carbonyl-group-containing impurities"). The resulting pentabromophenol containing carbonyl impurities has a large melting point range and coloration ranging from orange to dark orange and is therefore not a suitable final product.
Another scheme which has been utilized in the isolation and purification of perbrominated compounds includes the steps of an immediate initial filtration of the reaction mixture followed by washing the product one or more times with methanol to give a purified product. This isolation procedure has many advantages including simplicity. However, when applied to pentabromophenol the final product also contains the undesirable carbonyl impurities. Apparently, the carbonyl impurities are formed when the crude product, containing some free bromine, comes in contact with methanol during the washing step.
Accordingly, it is a primary object of this invention to obtain a process for the isolation of pentabromophenol that is superior to the techniques that heretofore have been employed.
Another object is to provide a process for producing purified pentabromophenol which avoids the formation of undesirable impurities during isolation.
A still further object is to provide a method of the character described that may be economically employed in isolating pentabromophenol.